FIG. 11 shows the internal structure of a prior art example of a color image forming apparatus, as disclosed in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246.
The printer comprises an intermediate transfer belt unit 101 including an intermediate transfer belt 102, a primary transfer roller 103, a secondary transfer roller 104, a cleaner roller 105, and a waste toner reservoir 106. Composition or superposition of color toner images is performed on the transfer belt 102. A group of image forming units 108 is made up of four image forming units 107Bk, 107Y, 107M and 107C, each unit being of sector shape in cross section. As can be seen in FIG. 11, the image forming units are arranged circularly in the middle of the printer.
When an image forming unit 107Bk, 107Y, 107M or 107C is set properly in the printer, mechanical and electrical connection systems are established between one of the image forming units 107Bk, 107Y, 107M and 107C and the machine body side via mutual coupling members. The image forming units 107Bk, 107Y, 107M and 107C are supported by a supporter, which is rotationally driven by a motor via a cylindrical shaft 109. Each image forming unit 107Bk, 107Y, 107M, and 107C is successively moved by rotation to an image forming position 110. The image forming position 110 is the position where a photosensitive drum 118 of the image forming unit faces the intermediate transfer belt 102 on the primary transfer roller 103, and is also the exposure position for exposure by a laser beam 111.
A laser exposing device 112 is provided in the lower part of the printer. The laser signal beam 111 from the laser exposing device 112 passes through an opening 113 between the image forming units 107M and 107C, and through an opening provided in the cylindrical shaft 109, and enters a mirror 114. This mirror 114 is positioned inside the shaft 109 and fixed directly to the machine body. The reflected laser beam 111 enters the image forming unit 107Bk located at the image forming position 110 through an opening 115, and passes through the space between a developing device 116 and a cleaner 117 of the image forming unit 107Bk, and enters an exposure portion of the photosensitive drum 118. The laser signal beam 111 is scanned by the exposing device in the direction of the axis of the photosensitive drum 118. The toner image, which is formed on the photosensitive drum 118 by exposure with the laser signal beam 111 and subsequent development with the developing device 116, is transferred to the intermediate transfer belt 102.
Then, the group of image forming units 108 rotates by 90 degrees, so that the yellow image forming unit 107Y moves to the image forming position 110 to replace the black image forming unit 107Bk. An operation similar to the operation explained above for the black image is performed to form a yellow image overlaying the black image formed on the intermediate transfer belt 102. Subsequently, the magenta and cyan image forming units 107M, 107C are moved to the image forming position 110, and similar operations as explained above are repeated to compose a full color image on the intermediate transfer belt 102. This full color image is further transferred from the intermediate transfer belt 102 onto a recording paper using a secondary transfer roller 119, and the image on the paper is fixed by a fixing device 120.
In the color image forming apparatus as explained above, precise registration of the toner images of all four colors is very important for obtaining a high quality full color image. However, an image forming unit system of the prior art as explained above, which forms a color image by overlaying successively four toner images of four photosensitive drums at one image forming position onto an intermediate transfer device (belt) and forms a color image on the recording paper by a secondary transfer from the intermediate transfer device, has the following disadvantage: After the start-up (power on), variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. Therefore, the rotational position of the intermediate transfer device at a predetermined time after the start-up is not necessarily a predetermined position. Consequently, it is necessary to ensure that sufficient time has passed so that the rotation of the intermediate transfer device is in a stable condition in order for a high quality image to be provided.
On the other hand, there is the strong desire to accelerate the recording operation. The time spent until the four image forming units comprising a photosensitive drum have been switched, and the rotation of the intermediate transfer device has been stabilized, is reflected in the time that the color image forming device needs for forming an image.
Moreover, the circumference of the intermediate transfer belt has an influence on the size of the entire device. Therefore, to make the circumference of the intermediate transfer belt as small as possible is desirable in order to miniaturize the device.
A main object of the present invention is to solve the problems mentioned above by providing a color image forming apparatus combining improvement of the image quality and the image forming speed with miniaturization of the entire device.